Feed-back elimination in alternating current tube sets



L. L.. JONES May 2, 1933.

FEED BACK ELIMINATION IN ALTERNATING CURRENT TUBE SETS .wml A Filed March 6, 1928 i ad Patented May 2, 1933 l UNITED STATES PATENT orFIcLE LESTER L. JONES, OF GRAD-ELL, NEW JERSEY FEED-BACK ELIMINATON IN ALTERNATING CURRENT TUBE SETS I Application elec March e, 192s. serial No. 259,453.v

This invention relates to radio receiving systems, and relates more particularly to highly sensitive radioA receiving apparatus;

l and has special reference to the provision of Y i means for controlling feed-back in radio receiving circuits employing alternating current sources of supply.

In 'building sensitive radio receiving sys- 1 tems, that is, receiving systems which have ka *0 large amount ofradio frequencyamplification, one of the'most difficult problems to 'solve is the control ork elimination of feedback of energy from a later stage of amplification to the input circuit of the radio frequency amplifier. This is especially true when the input circuit is of the loop type having minimum decrement'. l. Whenthe input circuit is made very sharply tuned, it requires only .the barest trace of amplified venergy .to create oscillation or signaldistortionl vThe channels through Ywhich these vdistant stage feed-backs, as I term them, occur are great in Vnumber and-many-of them have already been discovered and means devised for closing them. H i The present invention relates especially to the typ-e lof radio receiving system in which the amplifying tubes have their filaments en- 30 ergized by alternating` current and in which vthe detector tube is of the uni-potential cath- .ode or indirectlyheatedcathode type. In constructing a sensitive receiver of this type having, for example, six stages of radio frel quency'amplification' preceding the detector, 'If have discovered new-'hitherto unsuspected feed-back channels. The'prime objectof my present invention centers about the provision of means for controlling and for closing these feed-back channels. *i f f 'To the accomplishment-ofthe foregoing and such other objects as will hereinafter appear, my'invention consistsfin the elements and their relationl one to the other, as hereinafter more particularly described 'and sought to be' defined in the claims ;V reference being hadv to the accompanying drawing which shows thepreferred embodiment of my invention. and in which: F ig. l is a wiring diagram of a radio receiving system embodying the principles of my present invention, and

F ig. 2 isan explanatory wiring diagram of vessential parts thereof'.

Referring now more-in detail'to the drawing, and having reference first to Fig; l thereof, the radio receiving system to which my inventions are shown applied comprises a radio frequency amplifier having siX stages.

of radio frequency amplification employing electron relaysof thethree-element type designated-R. F. l to R. F. 6 inclusive, followed by an electron discharge tube detector designated also Det., the output of which feeds into an amplifier comprising two stages of audio frequency amplification, the first stage consisting of an electron relay tube .1 F. and the secondfstage consisting ofa push-pull' amplifier embodying two power4 tubes designated 2 A. F. l and 2 A. F. 2, the( output of kwhich containsterminals to which the speaker is connected. i

The radio frequency and audio frequency tubes, inthe embodiment of the invention herein shown, are of the usual hot filamentV type in which the filament is the electron emitter and the detector tube Det. is of the uni-potential vcathode typehaving a filament y, an indirectly heated cathode c, a grid g and a plate p. The filament f is vthe heaterv for the cathode and is energized by alternating current fed to the terminals designated 2.5V.Fil. The filaments of the, six radio frequency tubes R. F. l to R.F. 6 as well as the filament of the lirstaudio stage l A. F. are energized by raw alternating current, these filaments being `connectedin parallel and to a filament circuit F fed from the leads 10 connected to the terminals designated 1.5V-Fil. The filaments of the two tubes of the pushpull amplifier are connected in parallel and are energized by alternating current obtained from a source connected to the terminals designated l5V.Fil. The dial light for illuminating the receiver dial is also Vpreferably energized by this source, as clearly shown in the drawing.' All of the tube filaments farepreferably supplied from different windings on a single transformer, referred to further hereinbelow, `which forms part of a type.

cascaded by successively tuned and *unt-uned circuits, the tuned stages alternating with the untuned stages, all as described and claimed in my "Patent No. 1,620,661,dated March l5,

1927, to radio frequency amplifying system.

'j'Ihe untuned stages, as for example the stage intermediate the tubes R. F. 1 and B F. 2, or intermediate the tubes R. F. 3 and R. F. 1i,

`are constructed preferably as disclosed and claimed in my copending application Ser. No.

182,220,1'iled Apr. 9, 1927, to amplifying system. Volume adjustment is preferably secured by variably coupling the primary and seconda-ry of the tuned stages, as for example vthe stages intermediate the tubesRv. F. 2 and R. F. 3 and intermediate thetubes It. F. i and F. 5, as Ydisclosed and claimed in my copending application Ser. No. 123,346, filed July 19., 1926, to radio frequency amplifying system. The principles underlying the construction and arrangement of theR. F. amplifier are substantially fully `described in these copending applications and Letters Patent; and the constants of the various elements composing this amplifier are further specified hereinafter.

One of the essential objects of my present invention centers around the design of the fcircuits associated with A.. C. tubes of the uni-potential or indirectly heated cathode In the embodiment shown herein, this invention is applied to th-e detector tube Det.

Y but may be applied to A. C. tubes used either .for detection or radio frequency amplification purposes. T-he detector tube Det. has for its input a transformer L13, L14 wound biiilarly and including' a free section L15 coupled to ythe grid g of the detector'tube through a grid `condenser C10 and into the cathode c. vgrid leak R7 is also provided but the grid .condenser C10 and grid resistance R7 may be dispensed with at a slight loss of detector sensitivity. This radio frequency transformer L is encased 1n a separa-te metal case 12 which, as shown, 1s grounded at G with the cathode v0 of the detector tube.

I have discovered that in A. C. tubes of this character, there exists some unshielded capacity between the electrodes lof the tube, the

vgrid and pla-te of the tube inducing radio fre- .quency voltages on the heater filament f even vthough the cathode c apparently shields the heater from the grid and plate. I have found that there is some unshielded capacity, as for example in the stem of the tube, in the tube prongs, socket and adjacent lead wires;

`and with high R. F. amplification the voltages induced on the heater f from the grid and plate are suiicient to raise the potential of the heater circuit to a value high enough to create disturbing feed-back effects from the battery eliminator or power apparatus and its associated circuits back to the loop or antenna via the heater or filament supply line, the eliminator case or even the power Vsupplyv line connected with 'the eliminator.

I have empirically determined that the effects of this unshielded capacity resulting in the undesirable feed-back flow of energy may be controlled by suitably organizing the heater circuit; and more specifically, I have dis- -covered that the addition of the bypass con-v densers G15 and yC16 connected to each side A Vof thejheater supply line or circuit 13, 13

and connecting the same to ground potential prevents'the passage of radio -frequency current out along the heater supply line or pre- .vents the building up of potential abovev `ground (the metal case enclosing the receiver) Aby yproviding paths of very low impedance for the .radio frequency current from the heater terminals to the case or ground.

Preferably I employ two bypass condensers as shown, each oflow impedance,the use of the pair of condensers functioning not only for closing the feed-back channel, but also for eliminating a hum, particularly the modulation h um, which `modulation hum is dueto the modulation of the carrier wave by lsome inequality in the tube balance.

Thefheater or filament f lis preferably conductively vgrounded atsubstantially its midpoint and the feed-back control channels are preferably balanced by making the condensers C land C16 equal in capacity to a commercial degree of accuracy, as, .for example,

"approximately '5%. In Fig. 2 of the drawing is shown the manner in which the heater is thusconductively grounded, the filament supply line being taken from the .secondary l14 o-fa transformer, the said secondarybeing provided with a shunt potentiometer 15,

the mid-point of which is grounded by the l line 16. If only one bypass condenser should be employedyor if the lcondensers are not made equal in'capacityvwith this construction, the system tends to generate a hum audible inthe loud speaker and having a frequency approximately equal to Vthat of the upower supply frequency whenever there is present a strong carrier wave. This hum is eliminatedbyv the' feed-back control means l thus provided. The grounding of the cathode c of the detector tube, and preferably together with the grounding of the separate lshielding case 12,

is also of essential importance. With this construction I am enabled to devise a .simple ico currents generate.'

circuit-free from external radio vfrequency feed-backs. The grounding of the; cathode isting in the detector tube can circulate and -thereby reduces the voltages possible :from such circulatory currents, and also reduces the magnetic fields which such circulatory The provision of the metal case 12 serves .to prevent magneticfeedbacks from the R. F. transformer L13, L14,

L15 (Which transformer has the maximum .amount of radio frequency magnetic energy) to other coils and circuit connections at the input end of the system. It also serves to re- L duce the magnetic coupling to the loop through unavoidable cracks in the enclosing case of the radio system or When the receiver case is open fo-r adjustment.V c

lVhile `I have shown these inventions lap plied to the detector tube, the same may be applied, as heretofore stated, to a system of radio frequency tubesof the indirectly heated cathode type. lVhen a plurality of these tubes are used, the filaments of Which are connected in parallel circuit, a single pair of con- Ydensers corresponding to the condensers C15 and C16 may be employed for the entire filament supply line.

There the filament voltage is low, bypassing condensers C15 and C16 may be substituted or replaced by equivalent devices; for

vsuch low voltage filaments it becomes practical to substitute, for example, non-inductive resistances of fairly loW value for these bypass condensers C15 and C16. This would involve, however, a slight additional power loss due to current in this shunt resistance to the filament. The same substitution of equivalents may be made Wherethe invention is applied to a system of radio frequency tubes of the indirectly heated cathode type.

Preferably I provide a condenser C1 shunting the plate p and cathode c for bypassing radio frequency energy` from ,the

primary Winding of the audio transformer T1. This condenser 1s of relatively small capacity, namely, .002 mfds.,'so as not to shunt out the higher audio frequencies. As a. result some radio frequency'energy may be found at the lower end of the primarywinding of the transformer T1, and this is by.- passed to ground by the bypass condenser C111.

I have found that the filament supply line F for heating the filaments vof the radio frequency tubes also provides an undesirable channel for the feed-back of energy. from Va distant radio frequency stage back to the input, as, for example, the loop L1 of the B. F. amplifier. The existence of this feed-back channel is illustrated in Fig 2 of the ,drawing, wherein the tube feed-bach channels' across the vcapacityK of said tube-to and Athrough the input inductance L1 bac-k gto in the arroWed direction-through the filament line F to the filament of the first tube R. F 1 l70 ground Cr. For controllingl and'neutralizing this distant stage feed-back flow, the fila.- ment supply line F is organized so that the S radio frequency energy isk balancingly bypassed to ground. This I effect by arranging the filament supply line so that the output and input sides of the radio frequency amplifier are separated from each otherby a filter circuit consisting of bypass condensers-at the input and outputsides ofthe filament line and of an inductive reactance between them which is inherently provided bythe filament leads between the input and output sides of the amplifier. v z More specifically, as shown in Figs. 1- and 2 of the dravving,`at each end of the inductive reactance lilamentcircuit F,I. placethe bypass condensers C17 to C20 inclusive, jar

-pair of condensers C18, and C19Y being connected to the input side of the inductive reactance and another pair of condensers C17 and C20 beingconnected to the output sidev o f the inductive reactance. Preferablyl the filaments are rounded at their mid-points, this being edected by supplyingthe line F from the secondary 17` of` a transformer T4, the mid-point of Whichis tapped by the line 18 having a bias resistance 19./ With this arleo rangement, the condensers C18-G19v and .condensers are Aof relatively high capacity of the order of 1 mfd. each and are of the noninductive or quick dischargejtype. The center tapping of the transformer secondary 17 and the equalizing of the condenser capacities is desirablefor Vpreventing huml inthe receiver output. -l/Vithl this arrangement,

referring to Fig. 2 of the drawing, it will be .-115 seen thatthe radioy frequency potentials developed at both yends ofthe inductive reactance`A filament l line F are prevented,V ,from reaching the input kof the amplifier, the energy being bypassed to ground through the-low impedance channels provided therefor. Y

A small amount of radio frequency energy Y may also be carried through the audioffrequency amplifier system; and to compensate for this the condensers C12I and C13 are pro- 125 vided, these condensers functioning to bypass this radio frequency energy to ground so as to prevent its issue lfrom the receiver case into the loud speakerfromivhence it might feed back into ythe loop aerialgLl. i

y so

lThese condensers C12 and C13 are preferably of the order of .00 5 mfds. so as not to' bypass toomuch of the higher audio frequencies. The condenser C21 is also wiredin the circuit and to ground with the shortest possible leads, and this condenser is of the quick discharge type and of relatively high capacity. It serves to reduce'the impedance common to the plate circuits of the radio frequency tubes to a value sufficiently low to prevent feed-back through this channel.

. I have also foundr that some smalltraces of radio frequency energy may be found in the filament Circuit of the power tubes,'but this is insufiicient in a receiver of the sensitivity "shown'to produce lany appreciable detrimental effect.` After all of these feedback channels are closed, I have found that for' certain varrangements of power apparatus and loop some feed-backs might still occur, and I have traced these to magnetic coupling between the loop L1 and another loop consisting of the heater cables to the power supply device and from this back to the receiver through the B and C cable. This feed-back I have found may be avoided by combiningthese wires or cables in a single trunk or inside a single sheath. The filament wires to each individual tube are preferably transposedv by twisting to prevent the production of audio frequency magnetic fields which might bepicked up by the first audio transformer or by thevdetector input transformer. 'Y An example of the various magnitudes or constants of theelements of a radio receiving system built in accordance with the principles of the present invention and showing one of the preferred forms the invention may take, is as follows;

L-l loop-137 micro-henries L-5',L;9139 outerwinding 32 turns micro-henries inner winding 79 turns L-B, L-'7-185 turns #38 SSG Wire Y L-IO, L-1l*'90 turns bifilar #S8-DSE Wire L-12160 turns #38 SSC Wire .I1-13, L-ll-GO turns bifilar #38 DSE wire L-l, L-S-Bf turns :#:35 SSC Wire y@-2, C--.OOl micro-farads vC-, C-6-.OOO28 micro-farads C-8--00015 micro-farads C49, C--plO--OO025 micro-farads C-12, C-lS-YDOG micro-farads i Cari, 0 15, CAG, 0*17, C-is, o-ie, (3e-20, 'C-21=1 mrd. f

The use andy operation ofthe radio system of my invention and the many 4advantages thereof will in the main be'fullyr apparent from the above detailed'description thereof.

It willfurt-her be apparent that while I have Vshown and described my invention in the preferred form, many changes and modifications rnay be made 1n the structure disclosed Vwithout departing'from the spirit of the invention, defined inthe following claims,

VIclaim: Y 1. In a radio system, a radio frequency amplifier comprising-a plurality of radio frequency amplifier tubes arrangedin cascade, a circuit for the filaments of said amplifier tubes adapted to befed by alternating current, the filament circuit'between the input .and output ends of the amplifier forming an inductive reactance, and mechanism for controlling` the flow of an energy feed-back moving via said filament circuit comprising bypass condensers connected from each of the ends of said filament circuit or inductive reactance to ground at the input and output ends of the amplier. f l 3. In a radio system, a radio frequency amplifier comprising a plurality of radio frequency amplifier tubes arranged in cascade,

a circuit for the filaments of said amplifier tubes adapted to be fed by alternating current, and mechanism for controlling the flow of an energy feed-back moving via saidfilament circuit comprising low impedance bypass means connected from each of the opposite sides of said filament circuit to ground at both the input and output ends of said amplifier.

4. The combination of claim l in which;

the bypass means comprises a bypass condenser element connected from each side of said filament circuiti' to ground at both the inputand output ends of the amplifier. y

' 5. Ina radio system, a radio frequency amplifier comprising a plurality of radio frequency amplifier tubes arranged in cascade, a circuit for the filaments of said amplifier tubes adapted to be fed by alternating current, means for grounding the mid-point of said circuit, and mechanism for controlling the flow of an energy feed-back moving via said fila-ment circuit comprising four substantially equal bypass condensers, said condensers being connected from each-side of said filament circuit to ground at both the `input and output ends of said amplifier.

6. In a radio system, an electron discharge tube of the indirectly heated cathode type having grid, plate and cathode electrodes and a separate heater for the cathode, a circuit connected to the heater for supplying alter- *nating current thereto, a potentiometer type 1 condensers, one connected between each side of the heater circuit at a point near the tube directly to the nearest possible ground point, and each of said con-densers forming a direct low impedance radio frequency path therebetween, so as to substantially eliminate radio frequency potential on the heater with respect to ground.

7 In a radio receiving system, a plurality of radio frequency amplifying tubes and a detector tube arranged in cascade, said detector tube being of the indirectly heated cathode type having grid, plate and cathode electrodes and a separate heater for the cathode, a circuit connected to the heater for 1 supplying alternating current thereto, a pair of substantially equal bypass condensers, one connected from each side of said heater circuit directly to ground and forming a direct low impedance radio frequency path therebetween, a common supply circuit for the filaments of said amplifier tubes, and bypass condensers connected from each of the ends of said circuit to ground at the input and output ends of the amplifier.

8. In a radio frequency multi-stage amplifying system of high gain, a plurality of cascaded tubes at least one of which is an electron discharge amplifier tube of the indirectly heated cathode type having a grid,

i plate, and cathode electrodes and a separate heater for the cathode, a power supply source of heater current for said tubes, supply circuits from the source to the tubes for supplying current thereto, and means for controlling the iiow of a radio frequency energy feed-back moving through capacity channels in said tube and via said supply circuits comprising bypass condenser means connected from said cathode heater at a point adjacent the tube directly to the nearest possible ground point and forming a direct low impedance radio frequency path between the cathode heater and the said nearest ground point, said connection point adjacent the tube being at least nearer the cathode heater than the junction point of the supply source and the supply circuits running from said source to the other tubes of the amplifier, in order to substantially eliminate the radio frequency 'I potential on the heater with respect to ground.

9. In 'a radio frequency multi-stage amplifying system of high gain, a plurality of cascaded tubes at least one of which is an electron discharge amplifier tube of the indirectly heated cathode type having a grid, plate, and cathode electrodes and a separate heater for the cathode, a power supply source of heater current for said tubes, supply circuits from the source to the tubes for supplying current thereto, and means for controlling theV flow of a radio frequency energy feedback moving through capacity channels in said tube and via said supply circuits comprising a pair of bypass condensers, one connected from each side of said cathode heater at a point adjacent the tube directly to the nearest possible ground point and each forming a direct low impedance radio frequency path between the cathode heater and the said nearest ground point, said connection points adjacent the tube being at least nearer the cathode heater than the junction point of the supply source and the supply circuits running from the source to the other tubes of the amplifier, in order to substantially eliminate the radio frequency potential on the heater with respect to ground.

Signed at New York city in the county of New York and State of New York this 3rd day of March A. D., 1928.

LESTER L. JONES. 

